This invention relates to control apparatus for an adjustable hydraulic machine driven by a driving motor which can be set to different desired speeds.
For a drive assembly comprising an internal combustion engine and a continuously adjustable hydraulic (hydrostatic) transmission system and, optionally, further outputs or power take-offs, control apparatus has been proposed which comprises a fluid-operated adjustment motor having an adjusting piston which, under the action on one side of the piston of a first fluid control input whose pressure is a function of the actual speed of the internal combustion engine speed, is displaceable in one direction so as to effect the adjustment of an adjustable hydraulic pump or motor of the transmission system in one sense, the adjusting piston being displaceable in the opposite direction, under the action on an opposite side of the piston of a second fluid control input produced if the actual speed of the internal combustion engine drops by more than a predetermined amount relative to a desired speed to which the engine is set, so as to effect adjustment of the relevant hydraulic pump or motor in the opposite sense.
Control apparatus such as described in the preceding paragraph has been proposed in German Offenlegungsschrift No. 1,576,317. With this apparatus, the desired speed of the internal combustion engine is determined by a speed controller which, if the actual engine speed falls below the desired speed, channels the pressure of the first fluid control input on to the opposite side of the adjusting piston of the fluid-operated adjustment motor. Accordingly, the adjustment motor rapidly resets a hydraulic pump of the transmission system to zero stroke, i.e. the transmission ratio of the transmission system is reset, so as to avoid the overload of the internal combustion engine, until its actual speed has once again increased to the desired speed. In this way, regulation or control towards a predetermined working point, i.e. floating controller action, is achieved whereby the internal combustion engine is urged to regain its set desired speed with maximum rapidity. However, regulation or control arrangements of this kind tend to exhibit the phenomenon of "hunting".